Electric blasting cap



Jan. 9, 1940. I NASH 2,186,427

ELECTRIC BLASTING CAP Original Filed Dec. 22, 1937 HENRY E. NASH INVENTOR Wi -Mk 3. Mumm ATTORNEY Patented Jan. 9, i940 ELECTRIC BLASTING CAP Original application December 22, 1937, Serial No. 181,245. Divided and this application February 24, 1939, Serial Claims.

This invention relates to the production of an electric blasting cap and more particularly to an electric blasting cap with a very great de crease in the time necessary for firing.

It is well known that electric blasting caps do not-fire immediately upon application of the current, but require a. short interval of time between the application of the current and the detonation of the cap. This interval of time, to which we shall hereinafter refer as firing time, is most conveniently measured on an oscillograph, and at a given current has a definite value which is characteristic of the cap. At a low current, such as for example, 0.4 ampere, the firing time is relatively long, but as the current is increased, the firing time decreases, at first, rapidly, and then more slowly. For ordinary electric caps, such as those employed in quarrying and mining, the firing time cannot be decreased indefinitely. A limiting firing current is gradually approached so that, for example, at a current of about 5.0 amperes, the firing time of a cap containing mercury fulminate or diazodinitrophenol in the form of crystalline grains is approximately 0.002 second, which cannot be decreased to an appreciable extent by additional increase in the applied current.

Furthermore, it is observed that, at about 5.0 amperes, for example, the bridgewire or resistance wire which serves to ignite the explosive, may reach its fusion temperature in an interval of time shorter than that required to detonate the cap. In the absence of inductance or other efiects, the fusion of the bridgewire will interrupt the fiow of the firing current and it will then be observed that the opening of the firing line circuit and the detonation of the cap do not coincide.

These short time efiects are of little practical importance for the ordinary uses of the electric blasting cap, but in prospecting for oil or minerals by the seismograph method, they are of great importance. In the application of electric blasting caps and explosives to this method, the detonation serves to impart a tremor to the ground, which in traveling downward strikes the sub-surface hard'rock structure and is partially reflected. The time of arrival of the reflecting tremor at the surface is recorded, and knowing the instant of detonation and the mean velocity of the propagated wave, the depth of the refiecting structure can be calculated. The instant of detonation can be obtained by several methods, but the one in general use is that of taking the interruption of the firing line current of the electric blasting cap, as the instant of detonation. It will be obvious, because of the fusion of the bridgewire at high currents, that with the usual caps the recorded instant can difier from the true detonation time by an amount which will be nearly as large as the firing time of the cap. For seismograph types of work such differences in time will cause large errors in the location of underground structures.

With reasonably low firing currents or the addition of suitable inductance in the firing line circuit and the adjustment of circuit constants, it is possible to overcome the difilculties mentioned above, but such refinements are sometimes not very practical in the field. In the absence of such precautions, the errors in timing may be prevented by the use of caps containing faster firing compounds, such as lead styphnate or silver fulminate, for example. Lead styphnate has been used in blasting caps for many years and fast firing caps containing it have already found application in the seismograph method of prospecting. This compound, however, is expensive and difiicult to manufacture. Furthermore, it is sensitive to friction and, therefore, adds considerably to the hazards in the manufacture of such blasting caps.

Now, I have found that an electric blasting cap having a very fast firing time at high currents may be produced by using milled diazodinitrophenol admixed with a small amount of a binder and placing this mixture in the form of a paste immediately about the bridgewire and allowing it to form a relatively hard mass about said wire.

I have found particularly that by milling or grinding ordinary diazodinitrophenol, which has a grain size of about 150-350 microns, to a fineness such that at least the major portion of the milled material has a particle size range of about 1 to about 40 microns, preferably from about 20 to 40 microns, and then placing this suitably about the bridgewire, that I produce a blasting cap having a rapid firing time. When using diazodinitrophenol alone I find that it is necessary to incorporate a binder, such as, for example, nitrostarch, nitrocellulose, gum arabic, etc., dispersed in a suitable solvent, to form a mass around the bridgewire, in order to achieve a rapid firing time.

When using nitrostarch, for example, as a binder with milled diazodinitrophenol I first mill the diazodinitrophenol as a slurry in alcohol in order to obtain the desired grain size, then filter the ground material and dry it. Then as the last operation preparatory to filling the caps, a solution of nitrostarch in butyl acetate, for example, is added to the dried material and thor= oughly admixed therewith so that about 1% to about 4% of nitrostarch on the basis of the diazodinitrophenol is present. This paste is then worked about the bridgewire and dried, thereby forming a hard mass about said bridgewire. Larger amounts of a binder may be used but in general it will be found that the range above stated is the best for most practical purposes.

Alternatively, instead of milling in alcohol, drying and then adding the binder, I may mill directly in a solution containing the hinder, the concentration of binder in the solution being adjusted so that the amount of binder in the finished mixture is within the range above speci- A suitable blasting cap having fast firing properties can be made by placing in a metal shell a main charge, such as tetryl, pentaerythritol tetranitrate, a mercury fulminate-potassium chlorate mixture, or a like material and placing over this a booster or primer charge of ordinary grained diazodinitrophenol, lead azide, mercury iulminate, mercury fulminate-potassium chlorate mixture or a similar material, and then immediately over this and about the bridgewire vin the manner indicated above a small igniter charge composed of a milled mixture of diazodinitrophenol at least the major portion of which has a particle size range of about 1 to about 40 microns admixed with a small amount of a binder in the manner indicated before. The binder may be a nitrated carbohydrate, for example, nitrocellulose or nitrostarch or it may be a material such as gum arabic. Only a small amount of the milled diazodinitrophenol is necessary, because I have found that the firing time is determined by the ingredient immediately surrounding the bridgewire, the form or type of the charge in the main body of the cap being of little consequence.

In order to describe my invention more clearly I shall refer to the attached drawing which represents a vertical section of an electric blasting cap embodying this invention. It is to be understood, however, that this is done solely by way of illustration, and is not to be regarded as a limitation upon the scope of my invention.

Referring to drawing, the blasting cap comprises a metal shell l which may, for example, be made of copper, aluminum or the like. In the base of this shell is placed a main charge 2 which may be, for example, tetryl, pentaerythritol tetranitrate, a mercury fulminate-potassium chlorate mixture, or similar substances and superposed thereon a primer charge 3 which may, for example, be ordinary grained diazodinitrophenol, mercury fulminate or other similar materials. An igniter charge 4 comprising finely ground diazodinitrophenol at least the major portion of which has a particle size range from about 1 to about 40 microns admixed with a suitable binder is disposed in a suitable cavity 5 in the base of plug 6 which may, for example, be a sulfur plug. Above plug 6 is a waterproofing composition 1 of any desired composition and the shell I is closed at the top by means of a sulfur plug 8 which, when poured, enters the crimp Q.

and becomes keyed to the shell.

Lead wires I and II extend into shell I through plug 8, the waterproofing composition '7, and the plug 6 into the igniter charge 0. The ends of the lead wires are connected by means of greener a bridgewire 62, as is usual in cap construction.

Igniter charge 0, in the form of a paste, prepared in the manner before shown, is placed in cavity and worked about the bridgewire it. It is then allowed to dry and thereby forms a hard mass about the bridgewire. After igniter charge 0 has hardened, the remaining ingredients may be added and the finished cap prepared in the usual manner.

As a specific illustration of the very difierent and unusual characteristics introduced by finely ground diazodinitrophenol as above described, admixed with a binder, and placed about the bridgewire as before shown the following data are of interest. All of the caps were of the same general construction and strength containing the same base charge and priming charge and differed only in the ignition charge placed immediately about the bridgewire. All of the caps made were fired at a current of 5 amperes. The firing times as measured with an oscillograph are shown in the following table.

It will be seen from the above, that the finely ground diazodinitrophenol admixed with a binder, and placed about the bridgewire produces a cap having a remarkably fast firing time when compared with the cap in which grained diazodinitrophenol is used, in the form commercially used at present in regular blasting caps. In View of the fact that the firing time of the comparator is about as low as it has been possible to produce heretofore at a limiting firing current of about 5.0 amperes and which firing time the prior art has not been able to decrease to any appreciable extent, without resorting to expensive and difiicultly prepared compounds, even by an increase in the applied current, the results obtained with my caps, prepared as shown above, are Wholly unexpected.

It will be appreciated from the foregoing description and examples that the essence of my invention consists in placing about the bridgewire of a blasting cap having a suitable base and primer charge, a small charge of finely divided diazodinithrophenol at least the major portion of which has a particle size range of about 1 to about 40 microns admixed with a suitable binder in order to produce a relatively hard mass around the bridgewire as I have shown before. I am. able to produce in this manner a blasting cap which can be used in seismographic prospecting at high currents without producing appreciable errors in measurements. The present invention is, therefore, of great practical value in that it permits the production of a very fast-firing blasting cap by means of a well-known and commonly used explosive.

It will be understood that the details and examples given hereinbefore are illustrative only and in no way limiting on my invention as broadly described hereinbefore and in the appended claims.

aieaaa? This application is a division of my application, Serial No. 181,245, filed December 22, 1937.

What I claim and desire to protect by Letters Patent is:

1. An electric blasting cap including a shell, 2. detonating charge, a bridgewire and an igniter charge placed about said bridgewire comprising finely ground diazodinitrophenol at least the major portion of which has a particle size range of about 1 to about 40 microns admixed with a binder.

2. An electric blasting cap including a shell, a. detonating charge, a bridgewire, and an igniter charge placed about said bridgewire comprising finely ground diazodinitrophenol at least the major portion of which has a particle size range of about 1 to about 40 microns admixed with gum arabic.

3. An electric blasting cap including a shell, a detonating charge, a bridgewire, and an igniter charge placed about said bridgewire comprising finely ground diazodinitrophenol at least the major portion of which has a particle size range of about 1 to about 40 microns admixed with a nitrated carbohydrate.

4. An electric blasting cap including a shell, a detonating charge, a bridgewire, and an igniter charge placed about said bridgewire comprising finely ground diazodinitrophenol at least the major portion of which has a particle size range of about 1 to about 40 microns admixed with nitrostarch.

5. An electric blasting cap including a shell, a detonating charge, a bridgewire, and an igniter charge placed about said bridgewire comprising finely ground diazodinitrophenol at least the major portion of which has a particle size range of about 1 to about 40 microns admixed with nitrocellulose.

' HENRY E. NASH. 

